JP5363827B2 - Steel for machine structure, manufacturing method thereof and machine structure parts - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for machine structures which has excellent cutting treatability while maintaining its cold workability, a method for producing the same, and a component for machine structures. <P>SOLUTION: The steel has a composition containing, by mass, 0.005 to 0.045% C, 0.005 to 0.05% Si, 0.4 to 1% Mn, &le;0.05% P, 0.005 to 0.05% S, 0.01 to 0.06% Al and 0.009 to 0.02% N, and the balance Fe with inevitable impurities, wherein the solid solution amount of N is &ge;0.0085%, the structural fraction of a ferritic phase is &ge;90%, the average area per oxide inclusion in the whole structure is &le;2.5 &mu;m<SP>2</SP>, and also, the number of oxide and sulfide inclusions with the average area of &ge;0.5 &mu;m<SP>2</SP>per piece is &le;70 pieces per 0.25 mm<SP>2</SP>. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a machine structural steel manufactured into a machine structural component by cold working, a manufacturing method thereof, and a machine structural component.

  Bolts / nuts, pinion gears, steering shafts, valve lifters, common rails, and the like, which are parts for machine structures such as automobiles, have recently been required to have high strength for weight reduction.

On the other hand, mechanical structural parts such as crankshafts, connecting rods and transmission gears used in various gear transmissions such as transmissions and differentials for automobiles are generally subjected to hot working such as hot forging on steel. Then, it is finished to the final shape by cutting. Such mechanical structural parts are also required to be manufactured by forging by cold working instead of hot working in order to reduce CO ₂ emission in the manufacturing process.

  Unlike hot working, cold working is not a process under high temperature, and therefore, there is an advantage that these precisions are good because changes in shape and dimensions due to cooling are small. On the other hand, since the deformation resistance is high and the deformability is small compared to hot working, there is a problem that cracks are likely to occur in the steel material or the mold during working. Machine structural parts manufactured by cold working must not degrade workability.

  In addition, with the progress of cold working technology, the cutting allowance during cutting tends to decrease. As a result, when the cutting depth is reduced, the chips tend to stretch and become entangled, resulting in deterioration of the surface quality of the cutting product, and even forced to stop the automatic cutting operation. There arises a problem of chip disposal such as. In addition, after work hardening by cold working, the shear angle at the time of cutting increases, the chips become thinner, and the reduction in chip disposal becomes even more pronounced. Therefore, in the case of performing cutting after performing cold working without performing heat treatment such as quenching and tempering, improvement of chip disposal becomes a major issue.

  Therefore, as one of the means for improving the general chip disposability, there is a method of promoting the generation of MnS which is a soft inclusion by adding a large amount of S. By increasing MnS, the function of MnS as a stress concentration source becomes remarkable, and chips are easily divided. For example, Patent Document 1 proposes a high-sulfur free-cutting steel that contains a predetermined amount of a predetermined element and defines the contents of Mn, Cr, S, and O in a predetermined relationship. In the present invention, the machinability is maximized by suppressing coarse sulfides while increasing the S content.

In Patent Document 2, a predetermined amount of a predetermined element is contained, a B-based inclusion is not included as a nonmetallic inclusion, and a ratio of the C-based inclusion in the nonmetallic inclusion is 1% or more and less than 70%. A steel material excellent in cold forgeability and machinability is disclosed. In the present invention, C and S are added in order to add Ca and to obtain good cold forgeability in order not to generate Al ₂ O ₃ alone, which adversely affects chip disposal (tool wear characteristics). The range is limited.

  Furthermore, Patent Document 3 discloses a free-cutting steel characterized by containing a predetermined amount of a predetermined element, satisfying Al / N: 2.0 to 15.0, and not including a free-cutting element. Yes. In the present invention, by reducing the S content and limiting the Al and N contents to a predetermined range, AlN is precipitated in the steel, and the lubricating effect during cutting is exhibited.

JP 2005-307243 A JP 2002-322535 A JP 2003-226932 A

However, the above technique has the following problems.
In the high-sulfur free-cutting steel described in Patent Document 1, in order to spheroidize and finely disperse sulfide inclusions, a large amount of S is added, and the amounts of Mn, S, Cr, and O are within a predetermined range. Has been determined. However, since these elements are dissolved in steel to the extent that they are not used to form inclusions, and strengthen the steel solution, the solid solution strengthening increases the deformation resistance during cold working, thereby reducing the cold workability. There is a concern of deterioration. Moreover, since MnS to produce | generate also becomes a defect of steel materials, there exists a problem that the corrosion resistance of steel materials, hot or cold workability deteriorates. Furthermore, since sulfides such as MnS are soft, they tend to stretch in the forging direction (rolling direction) and cause anisotropy in material strength.

Moreover, in the steel material of patent document 2, although favorable machinability and cold forgeability are securable, the component strength after cold forging becomes the intensity | strength according to work hardening. Therefore, in order to reduce the weight and strength of the components, it is necessary to apply heat treatment, which increases the manufacturing cost and cannot contribute to CO ₂ reduction. Furthermore, in the free-cutting steel described in Patent Document 3, it is predicted that the tool protection effect by AlN is recognized, but there is a problem that the improvement effect of chip disposal and cold workability is not sufficient.

  The present invention has been made in view of the above-mentioned problems, and has as its object the steel for machine structure that is excellent in cutting processability while maintaining the cold workability, the manufacturing method thereof, and the machine structure thereof An object of the present invention is to provide a machine structural part obtained by processing steel.

In order to solve the above problems, the present inventors have examined the following matters.
In order to improve cold workability (cold forgeability), it is only necessary to reduce the interface between the hard phase (martensite, pearlite, bainite, etc.) and the soft phase (ferrite) that is the starting point of stress concentration during deformation. . However, when the hard phase is reduced, the steel becomes soft and the chips are easily connected for a long time, that is, the chip breaking property is lowered and the chip disposability is deteriorated. Thus, cold workability and chip disposal properties are contradictory.

  Therefore, in order to achieve both cold workability and chip breaking properties, it is considered that the size of the sulfide inclusions together with the oxide inclusions should be reduced and uniformly dispersed in the steel, and C (carbon) The amount was limited to the low C region. Thereby, the temperature range until molten steel solidifies can be narrowed, it can be solidified before the inclusion grows large, and a hard phase can also be reduced simultaneously. Subsequently, since heat is generated during cutting, it is considered that the soft steel itself should be hardened and brittle, so that a large amount of N is added to the steel and solid solution N (N in a solid solution state) is obtained by heat treatment. The amount was decided to increase. As a result, it became clear that chips are more easily divided than general carbon steel, and that the cold workability does not deteriorate at all. Furthermore, dynamic strain aging and solid strain aging due to solute N can improve part strength more than work hardening after cold working. Cold workability, post-cold working strength, chip disposal ( It was possible to arrive at the invention of steel for machine structural use (steel material) excellent in all of (chip cutting property).

That is, the steel for machine structure according to the present invention has C: 0.005 to 0.045 mass%, Si: 0.005 to 0.05 mass%, Mn: 0.4 to 1 mass%, P: 0.00. 05 mass% or less, S: 0.005 to 0.05 mass%, Al: 0.01 to 0.06 mass%, N: 0.009 to 0.02 mass%, with the balance being Fe and inevitable It has a composition comprising impurities, the N solid solution amount is 0.0085% by mass or more, the structure fraction of the ferrite phase is 90% or more, and the average per oxide inclusion in the entire structure The number of oxide-based and sulfide-based inclusions having an area of 2.5 μm ² or less and an average area of 0.5 μm ² or more per piece is 70 or less per 0.25 mm ^2. To do.

  Thus, by using ultra-low carbon steel, cold workability can be imparted with the main structure as the ferrite phase. Then, Mn is added for deoxidation and desulfurization during melting, while Al, which is also a deoxidizing element, is a solid solution amount of N so that Si, which is a deoxidizing element, does not deteriorate cold workability. In order not to reduce the amount, each is added in a small amount. Further, by sufficiently dissolving N, the strength of the soft ferrite structure can be increased more than the work-hardened content after cold working. Moreover, the balance between cold workability and machinability is improved by adding an appropriate amount of S. Furthermore, chip treatability improves by prescribing | regulating the average area etc. of an inclusion to predetermined.

Moreover, in the steel for machine structure according to the present invention, the composition may further contain one or more of Cr: 2% by mass or less and Mo: 2% by mass or less.
By adding Cr and Mo, the cold workability of the machine structural steel and the hardness after cold work are improved.

In the steel for machine structure according to the present invention, the composition further includes at least one of Ti: 0.2% by mass or less, Nb: 0.2% by mass or less, and V: 0.2% by mass or less. You may contain.
By adding Ti, Nb, and V, these nitrogen compounds are formed, the toughness after cold working of the steel for machine structural use is increased, and the crack resistance is improved.

In the steel for machine structure according to the present invention, the composition may further contain B: 0.005% by mass or less.
By adding B, a decrease in grain boundary strength due to segregation of P ferrite grain boundaries unavoidably contained is suppressed.

In the steel for machine structure according to the present invention, the composition may further contain one or more of Cu: 5% by mass or less, Ni: 5% by mass or less, and Co: 5% by mass or less.
By adding Cu, Ni and Co, the strain aging of the mechanical structural steel is promoted and the strength after cold working is improved.

In the mechanical structural steel according to the present invention, the composition further includes Ca: 0.05% by mass or less, REM: 0.05% by mass or less, Mg: 0.02% by mass or less, Li: 0.02% by mass. % Or less, Pb: 0.5% by mass or less, and Bi: 0.5% by mass or less.
By adding these elements, the cold workability of machine structural steel and the machinability after cold work are improved.

The manufacturing method of the steel for machine structure according to the present invention includes a melting step for melting the alloy having the composition described above, a casting step for casting the melt melted in the melting step, and a casting step performed in the casting step. A hot working step for hot working by rolling or forging the ingot at 1000 to 1200 ° C., and a hot work material hot worked in the hot working step from a state of 1000 ° C. to 200 ° C. A cooling step of cooling at a cooling rate of 2.5 ° C./s or more when cooling to a state .

  According to such a method for manufacturing machine structural steel, the machine structural steel described above, which is excellent in cold workability and chip disposal, is manufactured.

A machine structural component according to the present invention is manufactured by cold working the above-described machine structural steel at a start temperature of less than 200 ° C.
Such machine structural parts have good strength and hardness.

  The steel for machine structure of the present invention has sufficient cold workability and can be easily manufactured into a steel material having a certain size. In addition, it has excellent chip control when cutting with cemented carbide tools or coated tools, etc., so the surface quality of the cutting product will not deteriorate and the automation of cutting will be performed more stably. be able to.

  Moreover, in the manufacturing method of the steel for machine structures of this invention, the steel for machine structures excellent in the cold work property and the chip disposal property as mentioned above can be manufactured.

  Furthermore, the machine structural component of the present invention is cold worked using the steel for machine structural use of the present invention, cracks do not occur during cold working, the yield is improved, and the required strength and Indicates hardness. Therefore, it is possible to reduce the weight of the parts.

In some examples and comparative examples of the present invention, it is a graph which shows relation between part strength (hardness after processing) and maximum deformation resistance.

  Next, the steel for machine structure according to the present invention, the manufacturing method thereof, and the parts for machine structure will be described in detail.

≪Mechanical structural steel≫
The steel for machine structure contains a predetermined amount of C, Si, Mn, P, S, Al, N, and the balance consists of Fe and inevitable impurities. Furthermore, you may contain another component as needed.
Further, the solid solution N is a predetermined amount or more, the structure fraction of the ferrite phase, the average area per oxide inclusion in the entire structure, and the average area per piece is 0.5 μm ² or more The number of oxide-based and sulfide-based inclusions is predetermined.
This will be specifically described below.

<C: 0.005-0.045 mass%>
Since C is a ferrite single phase, it needs to be reduced as much as possible. However, if C is extremely small, deoxidation during melting becomes difficult. That is, if the amount of C is less than 0.005% by mass, gas defects are likely to occur during melting, and the yield deteriorates. Therefore, the C amount is 0.005% by mass or more. In addition, Preferably it is 0.01 mass% or more, More preferably, it is 0.015 mass% or more. Further, in the case of the steel for machine structure of the present invention, when the C content is 0.045% by mass, a structure in which fine cementite is slightly present substantially at the grain boundary of the ferrite single phase is obtained. However, if the amount of C exceeds 0.045% by mass, cementite will form pearlite, resulting in a ferrite-pearlite multiphase structure. Since pearlite is a hard phase, it deteriorates machinability and cold workability. Moreover, it takes more time to complete the solidification of the molten steel, and inclusions grow. Therefore, the C content is 0.045% by mass or less. In addition, Preferably, it is 0.043 mass% or less, More preferably, it is 0.04 mass% or less.

<Si: 0.005-0.05 mass%>
Si is an effective element as a deoxidizing element during melting. However, if the amount of Si is less than 0.005% by mass, the effect of deoxidation is not exhibited, gas defects are likely to occur during melting, and cracks are likely to start from there. Therefore, the amount of Si is made 0.005 mass% or more. In addition, Preferably it is 0.007 mass% or more, More preferably, it is 0.01 mass% or more. On the other hand, since Si strengthens the ferrite phase by solid solution, it causes an increase in deformation resistance and a decrease in cold workability. When the amount of Si exceeds 0.05% by mass, the tendency starts to be noticeable. Therefore, the amount of Si is made 0.05 mass% or less. In addition, Preferably, it is 0.04 mass% or less, More preferably, it is 0.03 mass% or less.

<Mn: 0.4-1 mass%>
Mn is an element effective as a deoxidizing and desulfurizing element during melting. Further, by combining with S, the deformability of the steel for machine structural use can be improved, and the effect of strengthening the ferrite phase by solid solution is obtained. However, if the amount of Mn is less than 0.4% by mass, the effects of deoxidation and desulfurization cannot be sufficiently exhibited, and cold workability starts to deteriorate. Therefore, the amount of Mn is 0.4 mass% or more. In addition, Preferably, it is 0.42 mass% or more, More preferably, it is 0.45 mass% or more. On the other hand, when the amount of Mn exceeds 1% by mass, deformation resistance due to solid solution strengthening is remarkably increased, so that cold workability is lowered. Therefore, the amount of Mn is 1% by mass or less. In addition, Preferably it is 0.98 mass% or less, More preferably, it is 0.95 mass% or less.

<P: 0.05% by mass or less>
P is an element inevitably contained as an impurity, but P segregates at the ferrite grain boundary and deteriorates cold workability. Further, P strengthens the solid solution of ferrite and increases the deformation resistance. Therefore, it is desirable to reduce as much as possible from the viewpoint of cold workability, but extreme reduction leads to an increase in steelmaking cost. Moreover, it is difficult on manufacture to set it as 0 mass%. Therefore, the P amount is 0.05% by mass or less (excluding 0% by mass). In addition, Preferably, it is 0.04 mass% or less, More preferably, it is 0.03 mass% or less.

<S: 0.005 to 0.05 mass%>
S is an element that is inevitably contained as an impurity, but when it is combined with Fe, it precipitates in the form of a film on the grain boundary as FeS, so that the cold workability is deteriorated. Therefore, the entire amount must be combined with Mn and precipitated as MnS. However, if the amount of S is less than 0.005 mass%, the machinability deteriorates. Therefore, the S amount is set to 0.005% by mass or more. In addition, Preferably it is 0.007 mass% or more, More preferably, it is 0.01 mass% or more. On the other hand, if the amount of S exceeds 0.05% by mass, the amount of MnS deposited increases, so that machinability is improved, but cold workability is deteriorated. Therefore, the S amount is 0.05% by mass or less. In addition, Preferably, it is 0.04 mass% or less, More preferably, it is 0.03 mass% or less. In this way, the upper and lower limit amounts of S were determined in consideration of the balance between cold workability and machinability.

<Al: 0.01 to 0.06 mass%>
Al is an effective element as a deoxidizing element during melting. However, if the amount of Al is less than 0.01% by mass, deoxidation during melting becomes insufficient and gas defects are likely to occur. Therefore, the Al amount is set to 0.01% by mass or more. In addition, Preferably, it is 0.015 mass% or more, More preferably, it is 0.02 mass% or more. On the other hand, if the amount of Al exceeds 0.06% by mass, Al ₂ O ₃ is easily generated during melting, and machinability is deteriorated. Moreover, since it couple | bonds with N and forms AlN, the amount of N solid solution is reduced and the component intensity | strength after cold working is reduced. Therefore, the Al amount is set to 0.06% by mass or less. In addition, Preferably it is 0.055 mass% or less, More preferably, it is 0.05 mass% or less.

<N: 0.009 to 0.02 mass%>
In the steel for machine structure according to the present invention, N (nitrogen) is dissolved in the steel and improves the strength after cold working (cold forging) of the steel for machine structure as described later. However, when the N content is less than 0.009% by mass, this N solid solution amount cannot be obtained sufficiently. Therefore, the N amount is set to 0.009% by mass or more. In addition, Preferably it is 0.0095 mass% or more, More preferably, it is 0.01 mass% or more. On the other hand, when the N amount exceeds 0.02 mass%, the N solid solution amount becomes excessive and the cold workability is deteriorated. Therefore, the N amount is 0.02% by mass or less. In addition, Preferably it is 0.018 mass% or less, More preferably, you may be 0.016 mass% or less. In addition, since N is inevitably mixed from the atmosphere in the steel melting step, the N content can be controlled by adjusting in the refining step. Moreover, you may add the nitrogen compound of the metal element (for example, Mn) contained as a component.

The steel for machine structure may further contain the following components as necessary.
<One or more of Cr: 2% by mass or less and Mo: 2% by mass or less>
Since Cr and Mo have the effect of improving the strength of the parts and the cold workability after cold working, they can be selectively added only in a predetermined amount. However, if the amount of Cr and Mo exceeds 2% by mass, respectively, the deformation resistance increases and the cold workability deteriorates. Therefore, the Cr and Mo amounts are each 2% by mass or less (excluding 0% by mass). In addition, Preferably, it is 1.5 mass% or less, More preferably, it is 1 mass% or less. On the other hand, in order to obtain the effect of addition of Cr and Mo, the Cr amount is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and further preferably 0.3% by mass or more. Moreover, 0.04 mass% or more is preferable and, as for Mo amount, 0.12 mass% or more is more preferable.

<Ti: 0.2% by mass or less, Nb: 0.2% by mass or less, and V: 0.2% by mass or less>
Ti, Nb, and V are effective elements for bonding with N to form an N compound to refine crystal grains, increase the toughness of parts obtained after cold working, and improve crack resistance. is there. However, since these elements have a strong affinity for N, if they exceed 0.2% by mass, N compounds are excessively formed and the amount of N solid solution is reduced. Accordingly, the Ti, Nb, and V amounts are each 0.2% by mass or less (excluding 0% by mass). In addition, Preferably, it is 0.15 mass% or less, More preferably, it is 0.1 mass% or less. On the other hand, in order to obtain the effects of addition of Ti, Nb, and V, the amount of Ti, Nb, and V is preferably 0.001% by mass or more, more preferably 0.002% by mass or more, and more preferably 0.003% by mass or more. Further preferred.

<B: 0.005 mass% or less>
B tends to gather at the ferrite grain boundaries, and is effective in suppressing a decrease in grain boundary strength due to P ferrite grain boundary segregation. However, since B has a strong affinity with N, when it exceeds 0.005 mass%, BN is formed, and the amount of N solid solution decreases, and BN segregated excessively at the ferrite grain boundary has a grain boundary strength. Reduce. Therefore, the amount of B shall be 0.005 mass% or less (excluding 0 mass%). In addition, Preferably it is 0.0035 mass% or less, More preferably, it is 0.002 mass% or less. On the other hand, in order to obtain the effect of addition of B, the amount of B is preferably 0.0002% by mass or more, more preferably 0.0004% by mass or more, and further preferably 0.0006% by mass or more.

<One or more of Cu: 5 mass% or less, Ni: 5 mass% or less, and Co: 5 mass% or less>
Cu, Ni, and Co are all effective in strain aging machine structural steel and improving the strength of parts after cold working. However, if the amounts of Cu, Ni and Co each exceed 5% by mass, the effect is saturated and cracking after cold working is also promoted. In addition, Preferably it is 4 mass% or less, More preferably, it is 3 mass% or less. On the other hand, in order to obtain the effect of adding Cu, Ni and Co, the amount of Cu, Ni and Co is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and more preferably 0.3% by mass or more. Further preferred.

<Ca: 0.05 mass% or less, REM: 0.05 mass% or less, Mg: 0.02 mass% or less, Li: 0.02 mass% or less, Pb: 0.5 mass% or less, and Bi: 0 .One or more of 5% by mass or less>
Ca, REM (rare earth metal element), Mg, and Li are elements that make sulfide inclusions such as MnS spheroidized, improve the cold workability of steel, and contribute to improvement of machinability. However, Ca and REM exceed 0.05% by mass, and Mg and Li exceed 0.02% by mass. Even when added excessively, the effect is saturated, and an effect commensurate with the amount added can be expected. It is economically disadvantageous. Accordingly, the Ca and REM amounts are each 0.05% by mass or less (excluding 0% by mass). In addition, Preferably, it is 0.03 mass% or less, More preferably, it is 0.01 mass% or less. The Mg and Li amounts are each 0.02% by mass or less (excluding 0% by mass). In addition, Preferably, it is 0.01 mass% or less, Preferably, it is 0.005 mass% or less. Specific examples of rare earth metal elements include elements such as Ce, La, and Nd, and the content of REM in this specification refers to the total content of all these rare earth metal elements.

  Pb and Bi are elements that contribute to improvement of machinability. However, when Pb and Bi exceed 0.5% by mass, problems in production such as rolling mills occur. Therefore, the amounts of Pb and Bi are 0.5% by mass or less (excluding 0% by mass), respectively. In addition, Preferably it is 0.4 mass% or less, More preferably, it is 0.3 mass% or less.

  On the other hand, in order to obtain the effect of addition of Ca, REM, Mg, Li, Pb, and Bi, the amount of Ca and REM is preferably 0.0005% by mass or more, more preferably 0.001% by mass or more, respectively. The mass% or more is more preferable. The amount of Mg and Li is preferably 0.0001% by mass or more, more preferably 0.0003% by mass or more, and further preferably 0.0005% by mass or more. The Pb and Bi amounts are each preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and even more preferably 0.05% by mass or more.

<Solution N: 0.0085% by mass or more>
N (solid solution N) dissolved in machine structural steel introduces many dislocations due to dynamic strain aging that occurs during cold working. After the cold working, the introduced dislocations are fixed by the solid solution N that has become easy to move due to the heat generated by the processing, thereby strengthening the static strain aging and increasing the strength more than the work hardening. . It is also effective for improving chip disposal. If the amount of N solid solution is less than 0.0085% by mass, the effect of increasing the strength by static strain aging cannot be obtained sufficiently. Moreover, since it becomes easy to connect a chip, chip disposal property tends to fall. Therefore, the N solid solution amount is set to 0.0085% by mass or more. In addition, Preferably it is 0.009 mass% or more, More preferably, it is 0.0095 mass% or more. On the other hand, if the amount of N solid solution is excessive, the influence of dynamic strain aging becomes more significant than static strain aging, and deformation resistance increases and cold workability deteriorates. Since the N solid solution amount is equal to or less than the N content in the composition, the upper limit value of the N solid solution amount is converged to the upper limit value of the N content, that is, 0.02% by mass. Such N solid solution amount satisfies the above-mentioned limitations on the N content and Al content, and as described later, the hot working (rolling, forging) temperature during production and the cooling in the cooling step. It is controlled by controlling the speed.

The value of the N solid solution amount is calculated according to JIS G 1228 by subtracting the nitrogen amount in all N compounds from the total N amount in steel.
The calculation of the total N amount in steel uses an inert gas melting method-thermal conductivity method. That is, a sample is cut out from the test steel material, put in a crucible, melted in an inert gas stream, extracted N, transported to a thermal conductivity cell, and the change in thermal conductivity is measured.

Calculation of the nitrogen amount in all N compounds in steel uses ammonia distillation separation indophenol blue absorptiometry. That is, a sample is cut out from a test steel material, and a 10% AA electrolyte (a non-aqueous solvent electrolyte that does not generate a passive film on the steel surface, specifically 10% acetylacetone, 10% tetramethyl chloride). (Ammonium, balance: methanol). Next, about 0.5 g of the sample is dissolved, and the insoluble residue (N compound) is filtered through a polycarbonate filter having a hole size of 0.1 μm. The insoluble residue is decomposed by heating in sulfuric acid, potassium sulfate and pure Cu chips and combined with the filtrate. After making this solution alkaline with sodium hydroxide, steam distillation is performed, and the distilled ammonia is absorbed by dilute sulfuric acid. Then, phenol, sodium hypochlorite and sodium pentacyanonitrosyl iron (III) are added to form a blue complex, and the absorbance is measured using a photometer.
The amount of N solid solution in steel is calculated by subtracting the amount of nitrogen in all N compounds from the total amount of N in steel determined by the above method.

<Ferrite phase structure fraction: 90% or more>
The mechanical structural steel according to the present invention has a soft ferrite phase as a main structure (substantially a ferrite single phase) in order to impart cold workability. By using a single-phase ferrite, when the machine structural steel is cold worked to produce machine structural parts, the entire structure is deformed and hardened uniformly at the same time, so the increase in deformation resistance can be suppressed as a whole. Cold workability does not deteriorate. Further, as a result of the examination, it is not always necessary to have a complete ferrite single phase structure, and it is sufficient that the area ratio (ferrite structure fraction) of the ferrite phase in the entire structure is 90% or more with respect to the entire structure. This is because even if cementite is precipitated at some grain boundaries, cold workability is not deteriorated if it is spheroidized. When the area ratio of the ferrite phase is less than 90%, the interface between ferrite and cementite tends to be the starting point of cracking, and cold workability is deteriorated. Therefore, the ferrite structure fraction is 90% or more. In addition, Preferably, it is 93% or more, More preferably, it is 95% or more.

  An example of a method for discriminating a tissue is observation with an optical microscope. In addition, as the position for observing the structure, the length in the cold working direction (compression direction) when manufacturing machine structural parts from the surface of the machine structural steel (when the diameter is reduced and processed into a cylindrical shape A position having a depth of ¼ of the diameter of the cylinder) is preferable, and a plurality of visual fields (for example, five visual fields) in the vicinity thereof are observed, and the average of the obtained tissue fractions can be determined. Specifically, machine structural steel is cut out so that the observation position is included in the cut surface, the cut surface is polished to a mirror surface, and then corroded with a nital solution (3% nitric acid ethanol solution) to optically corrode the corroded surface. Observed with a microscope at a magnification of about 100, the white area is the ferrite phase. In order to obtain the structure fraction, for example, a plurality of points (for example, 100 points) are randomly selected from the optical micrograph, the structure of each point is discriminated, and the percentage of the total number of points of the ferrite phase may be calculated. . Or you may process an optical micrograph with commercially available image analysis software, and may obtain | require the area ratio of a white area | region.

Here, in the case of a ferrite single phase structure substantially, the ferrite phase is generally softer than pearlite, martensite, etc., and chips are very easily connected (in terms of chip disposability, the ferrite single phase Multiphase structure is better than that). Therefore, the chip disposal can be improved by defining the inclusion as follows. That is, in the steel for machine structure of the present invention, the average area of oxide inclusions is 2.5 μm ² or less, and the number of oxide and sulfide inclusions of 0.5 μm ² or more is 0. By setting it to 70 or less per 25 mm ² , the chips are divided at a constant period.

<The average area per oxide inclusion in the entire structure is 2.5 μm ² or less>
If the average area per oxide inclusion exceeds 2.5 μm ² , the variation in cutting resistance increases and the chip breaking property deteriorates. Therefore, the average area is 2.5 μm ² or less. In addition, Preferably it is 2.2 micrometers ² or less, Preferably, it is 2.0 micrometers ² or less.

<The number of oxide-based and sulfide-based inclusions having an average area of 0.5 μm ² or more per one is 70 or less per 0.25 mm ² >
If the number of oxide-type and sulfide-type inclusions having an average area of 0.5 μm ² or more per piece exceeds 70 pieces per 0.25 mm ² , chip breaking property deteriorates and cutting resistance increases. Start. Therefore, the number is 70 or less. In addition, Preferably it is 65 or less, More preferably, it is 60 or less.

Measurement of inclusions can be performed, for example, as follows.
The sample after hot working is cut along a cylindrical axis and embedded in resin, and the cut surface is emery paper and diamond buffed so that the 1/4 position of the cylindrical diameter D and the vicinity of the axial center can be observed. Polish to a mirror surface. Next, automatic EPMA analysis is performed on the D / 4 position, thereby obtaining the composition, position, number of inclusions, area per inclusion, and the like.

≪Method for manufacturing steel for machine structure≫
The manufacturing method of machine structural steel includes a melting step, a casting step, a hot working step, and a cooling step.
Hereinafter, each step will be described.

<Dissolution process>
The melting step is a step of melting the alloy having the composition described above.
The method for melting the alloy is not particularly limited, and a conventionally known method may be used. For example, a vacuum induction furnace can be used.

<Casting process>
The casting process is a process for casting the melted material melted in the melting process.
The method for casting the melt is not particularly limited, and a conventionally known method may be used. For example, a continuous casting method or a semi-continuous casting method can be used.

<Hot working process>
The hot working step is a step of hot working by rolling or forging the ingot cast in the casting step at 1000 to 1200 ° C.
When AlN is generated during hot working, the amount of N solution decreases and desired characteristics cannot be obtained. Then, Al and N couple | bonded at the time of melting can be isolate | separated by setting the temperature at the time of hot processing to 1000-1200 degreeC. When the temperature during hot working is less than 1000 ° C., AlN cannot be decomposed sufficiently. Accordingly, the temperature during hot working is set to 1000 ° C. or higher. In addition, Preferably it is 1020 degreeC or more, More preferably, it is 1050 degreeC or more. On the other hand, when it exceeds 1200 ° C., it is sufficient for decomposing AlN, but the effect is saturated and the manufacturing cost increases. Accordingly, the temperature during hot working is set to 1200 ° C. or lower. In addition, Preferably it is 1180 degrees C or less, More preferably, it is 1150 degrees C or less.
The holding time can be determined according to the manufacturing conditions because AlN can be decomposed when the temperature is reached.

<Cooling process>
The cooling step is a step of cooling the hot-worked material hot-processed in the hot-working step at a cooling rate of 2.5 ° C./s or more in a temperature range of 200 to 1000 ° C.
In order to suppress the precipitation of AlN again during cooling after hot working, when cooling from a temperature range of 200 to 1000 ° C., that is, from a state of 1000 ° C. to a state of 200 ° C., 2.5 It is necessary to cool at a cooling rate of at least ° C / s. A lot of AlN is produced when it is slowly cooled or held between 200-1000 ° C. Therefore, at a cooling rate of less than 2.5 ° C./s, AlN precipitates and the N solid solution amount decreases. Therefore, the cooling rate is 2.5 ° C./s or more. In addition, Preferably it is 3 degrees C / s or more, More preferably, it is 3.5 degrees C / s or more. On the other hand, the upper limit is not particularly defined, but may be appropriately determined according to the manufacturing conditions. In addition, Preferably, it is 15 degrees C / s or less, More preferably, it is 10 degrees C / s or less.

  The method for manufacturing a machine structural steel according to the present invention is as described above. However, in carrying out the present invention, for example, casting is performed between or before and after each step within a range that does not adversely affect each step. Other processes such as a forging process performed between a process and a hot working process, a cutting process for cutting an ingot or a hot work material, and an unnecessary object removing process for removing unnecessary substances such as dust may be included. Good.

≪Mechanical structural parts≫
The machine structural component according to the present invention is manufactured by cold working (cold forging) the machine structural steel at a start temperature of less than 200 ° C. This is because if the start temperature is processed at 200 ° C. or higher, strengthening becomes insufficient. After the cold working, it is finished into a desired shape by a known method such as cutting.

The machine structural part obtained as described above is obtained by cold-working the machine structural steel at a starting temperature of less than 200 ° C. (from the state where the initial processing temperature is less than 200 ° C.) to obtain a part. The relationship between the component strength and the maximum deformation resistance during cold working satisfies the formula (1).
H ≧ (DR + 200) /2.5 (1)
In formula (1), H: strength of parts after cold working (Hv), DR: maximum deformation resistance (MPa) during cold working.

This formula (1) is constructed based on the following reason.
In general, when the strength of a part after cold working is increased, deformation resistance during cold working also increases. In the conventional product, for example, when the component strength after cold working is about 250 Hv, the maximum deformation resistance is about 500 to 550 MPa, and when the component strength is about 300 Hv, the maximum deformation resistance is low. When the component strength is about 350 Hv, the maximum deformation resistance is about 750 MPa at the lowest. In the present invention, compared with such a conventional product, the component strength is increased while suppressing deformation resistance, that is, the balance between the component strength after cold working and the deformation resistance during cold working is higher than that of the conventional product. The purpose is to obtain a machine structural component excellent in the above. Therefore, as a result of repeated investigations on the relationship between component strength and deformation resistance based on experimental data, the component composition and structure of mechanical structural steel are prescribed, and the mechanical structural steel obtained from this mechanical structural steel is used. Since the parts satisfy the formula (1), it becomes a machine structural part having a better balance between the strength of the parts after cold working and the deformation resistance at the time of cold working than the conventional product. It was a relational expression.

  Since the machine structural parts are obtained by cold working the machine structural steel described above, cracks do not occur during cold working, the yield is improved, and the required strength and hardness are obtained. It is shown. The machine structural component is manufactured by cold working (cold forging) the machine structural steel at a start temperature of less than 200 ° C. It is because strengthening becomes insufficient when processed at 200 ° C. or higher as in the case of cold working when manufacturing steel for machine structures. After the cold working, it is finished into a desired shape by a known method such as cutting.

Specific examples of cold-worked parts include steel for cold work for manufacturing bolts and nuts, pinion gears, steering shafts, valve lifters, common rails, etc., and cranks that have been machined by hot forging. A shaft, a connecting rod, a transmission gear, etc. are mentioned. The steel for machine structure of the present invention is useful as a material for producing these cold-worked parts by cold working (cold forging). Moreover, for the manufacture by cold working, it is possible to reduce emissions of CO ₂ in the component manufacturing process. These cold-worked parts have the required high strength even if they are reduced in weight.

  As described above, the present invention is excellent in the balance between the deformation load (resistance) applied to the mold when parts are processed by cold working and the strength of the parts after processing, and is based on a carbide tool or a coated tool. The present invention provides a machine structural steel that is excellent in chip disposal when it is cut, a manufacturing method thereof, and a machine structural component obtained by processing the machine structural steel, and has the following effects. It is what you play.

  (1) By refining coarse and hard oxide inclusions and sulfide inclusions, the cause of abrasive wear can be reduced and the burden on the tool can be reduced. (2) By limiting the size variation of oxide-based and sulfide-based inclusions, variation in cutting resistance during cutting can be suppressed, the load on the tool can be further reduced, and chips can be stabilized. Can be divided. (3) By containing solute N in a predetermined range and limiting the amount of C, it is possible to substantially form a ferrite single-phase structure, whereby the chips are rapidly embrittled by dynamic strain aging. The chips can be immediately separated from the work material. (4) By controlling the structure fraction of the ferrite phase, the cutting resistance does not fluctuate due to the hard phase such as bainite and martensite, and the cutting process can be consistently stabilized throughout. (5) The strength of parts after cold working is the amount of work hardening because a large amount of movable dislocations are introduced by dynamic strain aging during machining, and these dislocations are fixed by static strain aging after cold working. The above component strength can be obtained.

Examples of the present invention will be specifically described below in comparison with comparative examples.
[First embodiment]
Test material No. which consists of a component composition of Table 1 is shown. Sample steels 1 to 33 were prepared, 150 kg of the test steel was melted in a vacuum induction furnace, and cast into an ingot having an upper surface of φ245 mm, a lower surface of φ210 mm × a length of 480 mm. The ingot was soaked at 1150 to 1250 ° C. for 3 hours, hot forged into a 155 mm square material, cut to a length of about 600 mm, and a billet having a length of 155 mm square × 600 mm was obtained. Next, this billet was cut into a 155 mm square × 200 mm length and a 155 mm square × 400 mm length to form a steel piece, and each was welded to a dummy billet.

  Next, a steel piece of 155 mm square × 200 mm length was heated at the temperature shown in Table 1 together with the dummy billet, rolled (hot work) to a φ12 mm round bar, and cooled at the speed shown in Table 1. Furthermore, this round bar was cut, and a test piece having a length of φ10 × 15 mm was cut from the center (test piece 1).

  In addition, a 155 mm square × 400 mm long steel piece was heated together with the dummy billet at the temperature shown in Table 1, rolled into a round bar of φ80 mm (hot working), and cooled at the speed shown in Table 1. Further, this round bar was cut, and a test piece having a length of φ80 × 30 mm was cut out from the center (test piece 2).

Further, the N solid solution amount, the ferrite phase structure fraction (ferrite structure fraction), the average area of inclusions, and the like were measured by the following methods.
<N solid solution amount>
A sample cut out from the test piece was measured for the amount of N solid solution by an inert gas melting method-thermal conductivity method and an ammonia distillation separated indophenol blue absorptiometric method based on JIS G 1228.

<Measurement method of ferrite structure fraction>
The specimen is placed along the axis of the cylinder (half cylinder) so that the position at a depth of 1/4 of the diameter D of the cylinder from the surface of the test piece (hereinafter referred to as D / 4 position) and the vicinity of the center in the axial direction can be observed. Cut into shape) and embedded in resin, the cut surface was polished to a mirror surface with emery paper and diamond buff, and corroded with nital solution (3% nitric acid ethanol solution). The corroded surface was observed with an optical microscope to determine the structure and crystal grains. Tissue analysis was taken at 5 times (5 fields of view) at 100x, and these images were binarized using image analysis software (Image Pro Plus, Media Cybernetics) to produce white The area ratio of each photograph was calculated using the above region as the ferrite phase, and the average value of the five views was defined as the ferrite structure fraction.

<Measurement method for inclusions>
The specimen is cut along the cylinder axis (in a semi-cylindrical shape) and embedded in the resin so that the D / 4 position of the cylinder and the vicinity of the center in the axial direction can be observed from the surface of the test piece, and the cut surface is emery paper And polished to a mirror surface with a diamond buff. Next, automatic EPMA analysis (measurement elements: N, Na, Mg, Al, Si, S, K, Ca, Ti, V, Cr, Mn, and Fe) was performed on the D / 4 position. Thereby, the composition, position, number of inclusions, and area per inclusion were determined. In Table 1, the average area per oxide inclusion in the entire structure (referred to as the average area of oxide inclusions in the table), and the average area per piece is 0.5 μm ² or more The number of oxide-based and sulfide-based inclusions (denoted as the number of inclusions per unit area in the table).

  These test pieces 1 were evaluated for cold workability, and the test pieces 2 were evaluated for chip disposal.

<Evaluation of cold workability>
Each test piece (test piece 1) was compressed to 80% in the axial direction of the test piece by cold forging at a strain rate of 10 / sec at the starting temperature shown in Table 3 with a 1600t press and the end face restrained. Then, a processed test product (cold forging material) of a machine structural component was produced. The processing strain rate was an average value of strain rates during processing (plastic deformation). The compression ratio, when the compression direction length of the machine structural steel representing H _0, the compression direction length after compression (parts for machine structural) as H, calculated in _{(H 0 -H) / H 0} × 100 Is done. At the time of cold forging, a displacement resistance-displacement curve was recorded using a load cell attached to the 1600 t press and a displacement meter, and the maximum value of the deformation resistance in this curve was defined as the maximum deformation resistance. Moreover, the cold forging material which generate | occur | produced the crack by cold forging was evaluated as "*", and the cold forging material without a crack was evaluated as "(circle)".

  About each obtained process test article, the Vickers hardness of the cold forging material was measured as intensity | strength after cold work. Cut along the cylindrical axis of the cold forging material (the axis of the test piece before cold forging), embed it in the resin, adjust it as a sample, and adjust the diameter of the cold forging material at the center in the axial direction of the cylinder. A total of 6 points of Vickers hardness (Hv) were measured at 3 points on the left and right sides of the / 4 position.

<Evaluation of chip disposal>
Each test piece (test piece 2) was subjected to a drill cutting test to evaluate chip disposal.
The test conditions were as follows.

Tool: SKH51, φ5.0mm straight drill Cutting speed: 10, 15m / min
Feed: 0.1, 0.2mm / rev
Cutting oil: None (dry type)
Hole depth: 22 (mm) non-through hole

  After drill cutting, 100 chips were collected for each condition, and the chips were classified under the conditions shown in Table 2. In Table 2, as a representative, the test material No. 3, 6, 20, 22, 23, and 32 are shown. Based on the evaluation points shown in the table, the average of the four evaluation points was defined as the chip disposal index, and the quality of the chip disposal was determined based on the magnitude of this value. The chip disposability is indexed so as to increase as the chip becomes shorter. When the chip disposal index is 90 or less, the number of chips of 4 or less and 8 or less is increased. Therefore, 90 or more has good chip disposal, and less than 90 has chip disposal. Defect “x”.

  Specifically, the chip shape is 3 points for 1 roll or less, 2 points for 4 rolls or less, 1 point for 8 rolls or less, 0 for those exceeding 8 rolls, and a perfect score of 100. (If the chip form is 1 turn or less, it is a perfect score). For example, as shown in Table 2, the test material No. Taking 20 as an example, the number of chips of a steel type with a cutting speed of 15 m / min and a feed of 0.2 mm / rev is 71 for one turn or less and 29 for four turns or less. Therefore, (71 × 3 + 29 × 2 + 0 × 1 + 0 × 0) ÷ 3≈90.3. Similarly, when the feed 0.1 mm / rev is calculated, it is about 78.7. The average is (90.3 + 78.7) /2=84.5. When the calculation proceeds in the same manner, the average of the cutting speeds of 15 m / min and 10 m / min is 86.4. A chip disposal index of 20 is obtained.

  In these test results, when the obtained processed test article (test material) is not cracked and the deformation resistance is the same or low with respect to the Vickers hardness (hardness after processing) (specifically, When satisfying the condition shown in the formula (1)), it was determined that the cold workability was excellent, and when the chip disposability was good, the comprehensive determination was indicated as “◯”. On the other hand, when the crack occurred, when the deformation resistance was high with respect to the Vickers hardness, or when any one of the cases where the chip disposal was poor, the comprehensive judgment was displayed as “x”. These results are shown in Table 3. FIG. 1 shows the relationship between the component strength (hardness after processing) and the maximum deformation resistance in some examples and comparative examples of the present invention. In FIG. 1, the determination formula indicates Formula (1).

  As shown in Table 3, it was found that the test materials satisfying the scope of the present invention ensured good cold workability during processing and can obtain necessary hardness and strength after processing. Furthermore, it turned out that it is excellent in chip disposal property. That is, in the examples of the present invention (test materials No. 1 to 19), the overall judgment is “◯”, the obtained processed test product is not cracked, and has a low deformation resistance with respect to the Vickers hardness. It shows (specifically, satisfies the condition shown in the above formula (1)), exhibits excellent cold workability, and has excellent chip disposal.

  On the other hand, it turns out that the comparative example (test material No. 20-33) which does not satisfy any of said conditions is inferior to cold work property and / or chip disposal property.

That is, the test material No. No. 20 is an oxide having a high C content and an average area per oxide inclusion in the entire structure (hereinafter referred to as an average area of inclusions) of 0.5 μm ² or more. This is an example that deviates from the condition of the number of inclusions and sulfide inclusions (hereinafter referred to as the number of inclusions), cracking occurs after processing, deformation resistance is large, and deviation from the condition of formula (1) It shows a large deformation resistance against the Vickers hardness. Moreover, it is inferior to chip disposal property. Specimen No. No. 21 is an example in which the Si content is low, and cracks occur after processing. Specimen No. No. 22 is an example with a large Si content, and cracks occur after processing, which deviates from the condition of formula (1) and shows a large deformation resistance with respect to Vickers hardness.

  Specimen No. No. 23 is an example in which the Mn content is low, and cracks occur after processing. Specimen No. No. 24 is an example in which the cooling rate after hot working is low and the amount of N solid solution is small, and deviates from the condition of the formula (1) and shows a large deformation resistance with respect to the Vickers hardness. Moreover, it is inferior to chip disposal property. Specimen No. No. 25 is an example having a high Mn content, and cracks occur after processing. Specimen No. No. 26 is an example in which the P content is large, and cracks occur after processing. Specimen No. No. 27 is an example in which the S content is large, and cracks occur after processing.

  Specimen No. No. 28 is an example in which the Al content is large and the N solid solution amount is small, and deviates from the condition of the formula (1) and shows a large deformation resistance with respect to the Vickers hardness. Moreover, it is inferior to chip disposal property. Specimen No. No. 29 is an example in which the N content is low, the hot working temperature is low, and the N solid solution amount is low, which deviates from the condition of the formula (1) and shows a large deformation resistance with respect to the Vickers hardness. Moreover, it is inferior to chip disposal property. Specimen No. 30 is an example in which the hot working temperature is low and the amount of N solid solution is small, and deviates from the condition of the formula (1) and shows a large deformation resistance with respect to the Vickers hardness. Moreover, it is inferior to chip disposal property.

  Specimen No. No. 31 is an example in which the N content is large, and cracks occur after processing. Specimen No. 32 is an example in which the C content and Si content are large, the N content and the N solid solution amount are small, the average area of inclusions is large, the number of inclusions is large, and the ferrite structure fraction is low. In addition, cracks occur after the processing, the deformation resistance is large, and the deformation resistance deviates from the condition of the formula (1), and the deformation resistance is large with respect to the Vickers hardness. Moreover, it is inferior to chip disposal property. The test material No. No. 32 is a general ferrite-pearlite steel (S25C (ferrite fraction: 70 to 80%)) and has a low ferrite structure fraction. Specimen No. No. 33 is an example in which the N content and the N solid solution amount are small, and deviates from the condition of the formula (1) and shows a large deformation resistance with respect to the Vickers hardness. Moreover, it is inferior to chip disposal property.

[Second Embodiment]
Test material No. which consists of a component composition of Table 4 is shown. 34 to 48 test steels were prepared, 150 kg of the test steel was melted in a vacuum induction furnace, and cast into an ingot having an upper surface: φ245 mm × lower surface: φ210 mm × length: 480 mm. Next, each ingot was forged (soaking: about 1250 ° C. × 3 hr, forging heating: about 1000 ° C. × 1 hr) and cut into steel pieces of 150 mm square × 680 mm length. This steel slab was heated at the temperature shown in Table 4 and forged (hot-worked) into a φ80 mm round bar. Furthermore, this round bar was cut into about 350 mm lengths to obtain test materials. A test piece of φ10 mm × 15 mm was cut out from the D / 4 position, and a test piece of φ20 mm × 30 mm and φ30 mm × 45 mm was cut out from the central part (near the cylindrical axis) of a part of the specimen.
The N solid solution amount was measured by the same method as in the first example.

  The obtained three types of test pieces were evaluated for cold workability. The evaluation method is the same as in the first embodiment. The cold forging start temperature (° C.) is all 20 ° C. Table 5 shows the processing conditions, deformation resistance, presence / absence of cracks, and hardness after processing of each processed product. FIG. 1 shows the relationship between the component strength (hardness after processing) and the maximum deformation resistance in some examples and comparative examples.

  As shown in Table 5, it can be seen that the test materials satisfying the scope of the present invention can ensure good cold workability during the processing regardless of the part size, and can obtain the necessary hardness and strength after the processing. It was. That is, in the examples of the present invention (test materials No. 34 to 43), the overall judgment is “◯”, the obtained processed test product is not cracked, and has a low deformation resistance against the Vickers hardness. (Specifically, the conditions shown in the formula (1) are satisfied), and excellent cold workability is shown.

On the other hand, it turns out that the comparative example (test material No. 44-48) which does not satisfy any of said conditions is inferior to cold workability.
That is, the test material No. No. 44 is an example in which the N content is small and the N solid solution amount is small, and deviates from the condition of the formula (1) and shows a large deformation resistance with respect to the Vickers hardness.

  Specimen No. No. 45 is an example having a high C content and S content, and a low ferrite structure fraction. Cracking occurs after processing, which deviates from the condition of formula (1) and has a large deformation resistance with respect to Vickers hardness. Is shown. Specimen No. No. 46 is an example having a large C content, and cracks are generated after processing, deviating from the condition of formula (1) and showing a large deformation resistance with respect to the Vickers hardness.

  Specimen No. 47 is an example having a high Mn content and a low Al content, and cracking occurs after processing. Specimen No. 48 is an example in which the N content and the N solid solution amount are small, and deviates from the condition of the formula (1) and shows a large deformation resistance with respect to the Vickers hardness.

  As described above, the steel for machine structure, the manufacturing method thereof, and the machine structure parts according to the present invention have been described in detail by showing the best mode and examples. However, the gist of the present invention is limited to the above-described contents. is not. Needless to say, the contents of the present invention can be widely modified and changed based on the above description.

Claims (8)

C: 0.005-0.045 mass%, Si: 0.005-0.05 mass%, Mn: 0.4-1 mass%, P: 0.05 mass% or less, S: 0.005-0 0.05% by mass, Al: 0.01 to 0.06% by mass, N: 0.009 to 0.02% by mass, with the balance being composed of Fe and inevitable impurities,
N solid solution amount is 0.0085 mass% or more,
The structure fraction of the ferrite phase is 90% or more,
The average area per oxide inclusion in the entire structure is 2.5 μm ² or less, and
A steel for machine structure, wherein the number of oxide-based and sulfide-based inclusions having an average area of 0.5 μm ² or more per one is 70 or less per 0.25 mm ² .   The steel for machine structure according to claim 1, wherein the composition further contains at least one of Cr: 2 mass% or less and Mo: 2 mass% or less.   The said composition further contains 1 or more types in Ti: 0.2 mass% or less, Nb: 0.2 mass% or less, and V: 0.2 mass% or less, The Claim 1 or Claim characterized by the above-mentioned. Item 3. A structural steel according to Item 2.   The steel for machine structure according to any one of claims 1 to 3, wherein the composition further contains B: 0.005 mass% or less.   5. The composition according to claim 1, wherein the composition further contains one or more of Cu: 5 mass% or less, Ni: 5 mass% or less, and Co: 5 mass% or less. Machine structural steel according to Item.   The composition is further Ca: 0.05 mass% or less, REM: 0.05 mass% or less, Mg: 0.02 mass% or less, Li: 0.02 mass% or less, Pb: 0.5 mass% or less, And Bi: One or more types are contained among 0.5 mass% or less, Steel for machine structure as described in any one of Claim 1 thru | or 5 characterized by the above-mentioned. A melting step of melting an alloy having the composition according to any one of claims 1 to 6;
A casting process for casting the melted material melted in the melting process;
A hot working step of hot working the ingot cast in the casting step by rolling or forging at 1000 to 1200 ° C;
A cooling step of cooling the hot-worked material hot-processed in the hot-working step at a cooling rate of 2.5 ° C./s or more when cooling from a state of 1000 ° C. to a state of 200 ° C. A method for producing steel for machine structure, comprising:   A machine structural component manufactured by cold working the steel for machine structural according to any one of claims 1 to 6 at a starting temperature of less than 200 ° C.

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